There is a nitride semiconductor layer including In. The nitride semiconductor layer including In is used as, for example, a quantum well layer (hereinafter, referred to as a well layer) in a semiconductor light emitting device such as a light emitting diode and a laser diode. In the nitride semiconductor layer used for a semiconductor light emitting device, the crystal quality of the well layer is very important in order to improve the light emission efficiency. However, technology for forming a well layer of high quality has not yet been established. For the present, the nitride semiconductor layer including In has the problem of segregation of In. For example, a region of the well layer where the composition ratio of In is high is weak in thermal stability, and may form a starting point of a crystal defect such as a phase separation and reduce the light emission efficiency. Technology for manufacturing a nitride semiconductor layer of high crystal quality which provides a well layer with high light emission efficiency is desired.